I bought on Steam and was provided with a key by Steam when the installation was complete. It prompted me that the key might be needed when the game was started. So I copied it to the clipboard to have it ready if needed.

I started the game and there was no request for the key. Everything went smoothly. The next time I started the game, Steam again provided the key. There was a checkbox "don't show me this again". Since everything was working fine, I clicked on that checkbox and have not been given the key since.